3A molecular sieve

What is molecular sieve?

A synthetic silico-aluminate hydrate (bubble zeolite) or natural zeolite having the function of screening molecules. Its general chemical formula is (M '2M)O·Al2O3·xSiO2·yH2O, M', M are univalent and bivalent cations such as K+, Na+ and Ca2+, Ba2+, etc. It has a lot of uniform pore size and arranged holes in the structure, different pore size of molecular sieve separated molecules of different sizes and shapes. According to the different molecular ratio of SiO2 and Al2O3, different pore sizes of molecular sieves were obtained. Its models are: 3A(potassium A type), 4A(sodium A type), 5A(calcium A type), 10Z(calcium Z type), 13Z(sodium Z type), Y(sodium Y type), sodium mercerite type, etc. It has high adsorption capacity, strong selectivity and high temperature resistance. Widely used in organic chemical industry and petrochemical industry, it is also an excellent adsorbent for gas dehydration. More and more attention has been paid to the purification of waste gas.

introduction

A natural silico-aluminate exists in nature that has sieve molecules, adsorption, ion exchange, and catalysis. This natural substance is called zeolite, synthetic zeolite is also called molecular sieve. The general formula of the chemical composition of molecular sieve is: (M)2/nO· Al2O3·xSiO2·pH2O, M represents metal ions (usually Na when synthetic), n represents the valence number of metal ions, x represents the mole number of SiO2, also known as the ratio of silicon to aluminum, p represents the mole number of water. The basic structure of molecular sieve skeleton is SiO4 and AlO4 tetrahedrons, which form a three-dimensional mesh structure through the combination of common oxygen atoms. This combination form, with molecular level, uniform pore size of the cavity and pore channel.

Since the AlO4 tetrahedron has a negative charge, it can bind to the sodium plasma and become electrically neutral. In an aqueous solution, Na is readily exchanged with other cations. Most molecular sieve catalysts are polyvalent metal cations or H exchange, molecular sieve is acidic and selective to molecular size, can be used as a catalyst or carrier. High silica zeolites exhibit a high affinity for organic groups, whereas low silica zeolites exhibit hydrophilicity due to their Lewis and Bronsted acid properties. Silicon and aluminum atoms form oxygen ring through oxygen, and the size of oxygen ring determines the pore size of zeolite. The number of oxygen atoms in each oxygen ring ranges from 4 to 12. The molecular sieve is usually used as eight-membered ring (0.4 ~ 0.5nm), ten-membered ring (0.5 ~ 0.6nm) and twelve-membered ring (0.7 ~ 0.9 nm).

There are Y molecular sieve (x= 3.1 ~ 6.0) and mercerite (x=9 ~ 11) with twelve binary oxygen rings. The former can be used as cracking catalyst, bifunctional catalyst, the latter can be used as disproportionation catalyst of toluene.

The ten-membered oxygen ring contains ZSM-5, ZSM-11 and other ZSM series molecular sieves.

The octonal oxygen ring includes A type molecular sieve (x=2), T type molecular sieve and ZSM-34. Their pores are so small that only straight-chain hydrocarbons can enter the pores. The catalyst with molecular sieve as the catalytic active component or the main active component is called molecular sieve catalyst. Molecular sieve has ion exchange performance, uniform molecular size pore, excellent acid catalytic activity, and has good thermal stability and hydrothermal stability. Catalysts with high activity and selectivity for many reactions can be prepared.